You always feel foolish for the first few months in a new project, because you have no context for what is going on, where things are, and what you should be doing most of the time. This is not a personal failing, nor is it usually a failing of the project (although there is a lot current developers can do to make a project more welcoming to newcomers) — one simply cannot magically obtain all of the knowledge current developers have internalized in an instant, no matter how hard one tries. This is true even when you aren't entry level anymore, although I can't speak to the experience of truly senior (= multiple decades of experience) people.
Remember also that the efficiency of onboarding depends much more on the existing team (and former members) than it does on the new developer. Good documentation and clear instructions make the process much smoother; chaos and a lack of easily digestible written information will make the process longer and more painful. When you are on the team and onboarding a new developer, do not forget this. When you are the new developer and run into a lack of clear documentation, remember that it is not your fault — but be sure to write down all the things that you wish were documented when you started, so that you can write the missing documentation later!
So don't feel bad about yourself if it takes a while to get up to speed on your new project. It doesn't say anything bad about you, although it may reveal points for improvement in the existing project's documentation. Just spend some time digging, ask questions when you can't figure something out, and you will eventually get to know your way around.